PAPERS ON BOTANY 229 



Another diffienlty was in identifying the difficnlt species 

 of oaks. The red oaks are so mnch alike that it is al- 

 most impossible to distinguish between them without 

 special study. An investigation is being carried on, at 

 the present time, by some of the students of Xorthwest- 

 em University, under the direction of Dr. W. G. Water- 

 man, to determine the relative proportions of Quercus 

 rubra, Q. velutina and Q. ellipsoidalis in this vicinity. 

 The white oaks of the region, Quercus alba, Q. macro- 

 carpa and Q. bicolor are more distinct species, though 

 even with them identification is sometimes uncertain 

 in individual cases where leaves and acorn cups can not 

 be had for comparison. But in general, the reported 

 distribution of the species of white oaks may be consid- 

 ered reasonably correct. 



Phtsiogbapht 



a. geology 



The whole of the region, with the exception of the 

 portion west of the Gross Point Eoad and its extension 

 north, is located upon the Chicago plain, which was once 

 a part of the floor of Glacial Lake Chicago. The history 

 and description of this plaiQ are given fully by Bannis- 

 ter (3), Leverett (9), Salisbury and Alden (10) and es- 

 pecially by Atwood and Goldthwait (2). so that only the 

 essential features need be discussed here. 



As the continental glacier receded from the Lake Mich- 

 igan trough and the country round about, it appears 

 to have left at its foot a large body of water which ex- 

 tended over a greater territory than the lower end of 

 Lake Michigan and is appropriately called Lake Chicago. 

 Due to possible tilting, changes in outlet and other causes 

 the recession of the lake was not gradual but there were 

 three stages at which the waters of the lake stood for a 

 long time, each stage having its trace in beaches and 

 bars on the lake plahi as it was exposed. 



At the earliest, or Glenwood stage, the water stood 

 sixty feet higher than the present level of Lake Michi- 

 gan. The beach can be traced from the water works 



